There is a need to determine directions, angles and lengths as positions in many fields of application, such as in geodetic and industrial measurement, for example. Developments in angle measurement technology have led via mechanical reading operations as far as fully automated angle measurement in accordance with the current state of the art.
Known automated position measuring devices generally comprise a code carrier and a scanning device. In angle measuring devices, the code carrier is usually designed to rotate about an axis relative to the scanning device, in which case an angular position of the code carrier constitutes the variable to be measured. The coding carrier can, for example, have a graduation or coding for determining the position, it being possible to apply the coding to a surface or lateral surface of the coding carrier.
For the purpose of automatically detecting the position, the code carrier, which can be moved relative to the scanning device, is scanned by means of differing techniques. Known scanning methods are electronic-magnetic, electronic and optoelectronic methods. The following statements relate to optoelectronic scanning methods and scanning devices which have, in particular, an illumination device and a detector.
The coding is usually arranged in a full circle in order to determine, for example, angular positions from 0° to 360°. The angular resolution of the full circle is determined by the type of coding and the scanning device used to read the coding. Thus, for example, the angular resolution is increased by applying a code in a plurality of tracks, or by a finer graduation, the achievable resolution being limited for reasons of manufacturing and cost. Arrangements of one or more detectors, for example, are known for reading the code. CCD line arrays or CCD area arrays, for example, can constitute such detectors. The code can be formed by structuring a reflecting surface, or else by structuring a transilluminable material so that imaging is performed in transmission, reflection or in a combined method.
Swiss patent CH 658514 A5 discloses such a device for measuring an angular position. There, a mark whose position relative to a surface of sensors constitutes the variable to be measured is focused onto this surface. The output signals of the sensors are guided into an evaluation circuit that sequentially determines the distribution of the intensity of the signals generated by the sensors. The position of the mark in relation to the surface of sensors can be derived from the intensity distribution.
The dimensions of a position measuring device for geodetic units are advantageously to be kept small. In order to permit a design that is correspondingly small and not very complicated, the illumination device and the detector of the position measuring device have for some time been arranged on a common, electrically supplied printed circuit board and not, as in the past, above or below a code carrier, this being done in each case on a separate printed circuit board that was electrically supplied. In the case of position measuring devices of the prior art having a side-by-side arrangement of the detector and the illumination device, the emitted beams are deflected, for example, by a deflecting element having two planar, reflecting surfaces, so that an image of the code is produced on the detector by the code carrier arranged downstream in the beam path. In this case, the emitted beams can optionally be collimated by means of an optical system directly downstream of the illumination source.
By way of example, U.S. Pat. No. 7,145,127 B2 shows a deflecting element for a position measuring device, the deflecting element having two mutually aligned surfaces for double reflection of the emitted beams, and carrying a code that can be imaged onto a sensor, and thus additionally being formed as a code carrier.
One object of the invention is therefore to provide a position measuring device having a simplified design, as well as a position measuring method suitable herefor.
A further object is to reduce the size and to increase the robustness of the position measuring device.
These objects are achieved by implementing the characterizing features of the independent claims.
Features that develop the invention in an alternative or advantageous way are to be gathered from the dependent patent claims.
The invention is based on a fundamental variation in the beam path used to illuminate the position code. In accordance with a first partial invention, the beam guidance is displaced into the code carrier in this case. This first configuration of the code carrier, which is independent of the second invention, is also described in the European patent application with the application number 09151945.5, and defined in the claims, the disclosure of which is incorporated into this description by reference. In accordance with a second partial invention, which develops this first partial invention, the decoupling of the radiation from the code carrier is configured so as to perform illumination of a detecting element with homogeneous intensity.
In the prior art, the illumination of the position code is performed in accordance with transmitted light or reflected light methods by guiding the radiation onto the code carrier as perpendicularly as possible. Depending on the particular configuration, this requires deflecting elements or complicated beam paths, the result being that in addition to the size of the device required for this purpose, there is also an increased outlay on adjustment and a reduction in robustness. According to the invention, the radiation used for illumination is coupled into the code carrier itself and guided there at least in sections, the beam path lying at least partially in the plane of extent of the code carrier and, in most cases, also of the position code. This means that the propagation direction of the radiation and the optical axis of the beam path lie in this plane, thus enabling a flat design despite the use of a planar code carrier or of a code extended over an area or linearly. Beam guidance is preferably performed up to the position code itself, so that the latter is illuminated from out of the code carrier.
The materials glass or plastic used generally in the prior art can be used for this purpose as suitable code carrier materials in which beam guidance can be performed by reflection at the interfaces. However, it is also possible in principle to make use of cavities, for example made from metal, reflection being performed here at the inner surfaces.
For the application purpose of a goniometer or rotation encoder, it is possible, for example, to use a plastic circle or plastic ring with an impressed diffractive code such as is described in WO 2008/141817, for example. The illumination radiation is coupled through the narrow side into this plastic circle from inside or outside with the aid of a light source. The graduation representing the position code comprises alternately occurring disturbed and undisturbed parts on the surface of the disk, formed from diffractive, refractive, absorbing or reflecting structures. Consequently, the position code can, for example, be designed in reflection or transmission as a phase grating or amplitude grating. The receiver is provided as an area array or line array on the side of the circle on which the surface is partially disturbed or, given an appropriate design, also on the other side, a plurality of arrays and light sources being capable of use in order to determine or eliminate systematic errors such as, for example, eccentricity errors, or to increase the measuring accuracy. Such high-precision angle sensors are typically applied in geodetic measuring units or scanning measuring machines such as coordinate measuring units, for example.